1. Field of the Invention
The present invention relates to an analog-to-digital converter and, particularly, to an analog-to-digital converter having a calibration function of a comparator that converts an analog value to a digital value.
2. Description of Related Art
In the signal processing field, signal processing based on digital signals has become mainstream today. Thus, if an externally input signal is a signal having an analog value (which is referred to hereinafter as an analog signal), the analog signal is converted to a digital signal and processed. One of circuits that perform such conversion processing is an analog-to-digital converter. The analog-to-digital converter includes a comparator that compares the value of an input analog signal with the value of a reference signal and converts the analog signal to a digital signal. The comparator, however, generally has an offset. Due to the offset, in the comparator, a threshold for inverting the logic level of an output signal is deviated from the voltage value of a reference voltage. In order to perform analog-to-digital conversion with high accuracy, it is necessary to reduce the deviation of the threshold. One approach to eliminate the threshold deviation is to reduce the threshold deviation of the comparator by calibration processing.
An example of a technique of performing such calibration processing is disclosed in U.S. Pat. No. 7,046,179. FIG. 20 is a block diagram showing an analog-to-digital converter 100 disclosed in U.S. Pat. No. 7,046,179. The analog-to-digital converter 100 shown in FIG. 20 includes a calibration reference voltage generator 130, a selector 145, a sample-and-hold circuit 160, conversion circuits 121 and 122 and a control circuit 140. During a calibration period, the analog-to-digital converter 100 performs calibration of the converter 100 by using a calibration reference voltage CALREF that is generated in the calibration reference voltage generator 130. The analog-to-digital converter 100 can thereby correct an error of the converter 100 and improve the conversion accuracy.
Japanese Unexamined Patent Publication No. 2008-54205 discloses an example of an analog-to-digital converter that improves the conversion accuracy without performing the calibration processing.